Unfortunately you do not write since when the contract is running and you do not write the exact tariff name.

 

1. the 3 months period is no more valid, has been legally replaced  on Deceber 1st with a 1 month period; without cancellation or active prolongation from your side the contract is (after the initial 24 months) renewed automatically with an undefined end
2. undefined end, can be terminated within a one months notice once the initial 24 months are over
3. if you mean prepad then you explicitely need to cancel the contract and following that actively change to a prepaid plan (tariff change)
4. the phone is yours

I guess that you do not want to stay in your current tariff plan because that probably includes a monthly payment on top for the smartphone - and if you do not request a new smartphone (that would trigger a new 24 months contract duration) then it is better to change to a tariff variant "without smartphone".

 

Assuming that you have basically two options

1. to the end of your current contract duration you change to the "flex" tariff variant in the "ohne Smartphone" regime
2. you cancel the contract and drive a tarif change to a prepaid tariff

There are some differences between the two, e.g. in 1) you have more features, like e.g. an optional multi-SIM for a smart watch or in 2) you can (for example out of a temporary personal money shortage) simply not recharge, so no payment from your bank account is necessary for several months (when Telekom will cancel the prepaid contract due to no more recharging), of course with the consequence that you can no more actively use the SIM, just receive calls and text messages